Experimental analysis of the thermal management and internal quantum efficiency of terahertz quantum cascade laser harmonic frequency combs.

Quantum cascade laser (QCL) harmonic state frequency combs (HFCs) have recently attracted considerable interest for applications ranging from the generation of tones of high spectral purity, high data rate wireless communication networks, radiofrequency arbitrary waveform synthesis, and for fundamental light-matter investigations in quantum physics. However, a detailed knowledge of the nature of the electronic and thermal energy distribution in these devices is of paramount importance for the refinement of their thermal management and quantum efficiency, which are key to the widespread adoption of QCL HFC technology in a new generation of integrated optical quantum platforms. Here, we perform a comparative study of the thermal and electronic properties of Fabry-Perot and micro-ring HFC QCLs, operating in the terahertz frequency range, using micro-probe band-to-band photoluminescence. By monitoring the lattice temperature and the electron cooling above the threshold for stimulated emission, we extract the device thermal resistances and the internal quantum efficiencies, highlighting the key role of the resonator architecture.